Re: [PATCH v2] libstdc++: Remove unnecessary code from reduce implementation

On Tue, Aug 18, 2026 at 10:10 AM Matthias Kretz <[email protected]>
wrote:

> Resending (see patch from 2026-07-02). OK for trunk?
>
> ------------- 8< -----------
>
> From: Matthias Kretz <[email protected]>
>
> Previously, this was necessary for efficient reductions. When I tried to
> turn it into a missed-optimization PR, there was no issue (anymore). The
> generic _S_static_permute implementation optimizes just fine (or better)
> at this point.
>
> The traits_impl.cc change is a simple drive-by fix.
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/simd_vec.h (_M_elements_shifted_to_front): Remove
>         x86-specific implementation.
>         * testsuite/std/simd/traits_impl.cc: Fix incorrect macro name.
>
> Signed-off-by: Matthias Kretz <[email protected]>
>
Thanks, this LGTM. Happy to see that optimization is no longer needed.
Still needs Jonathan ACK.

> ---
>  libstdc++-v3/include/bits/simd_vec.h          | 32 +++----------------
>  .../testsuite/std/simd/traits_impl.cc         |  2 +-
>  2 files changed, 6 insertions(+), 28 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/simd_vec.h
> b/libstdc++-v3/include/bits/
> simd_vec.h
> index bf6616df227..9473c57b6cd 100644
> --- a/libstdc++-v3/include/bits/simd_vec.h
> +++ b/libstdc++-v3/include/bits/simd_vec.h
> @@ -449,33 +449,11 @@ _M_elements_shifted_to_front() const
>           static_assert(_Shift < _S_size && -_Shift < _S_size);
>           if constexpr (_Shift == 0)
>             return *this;
> -#ifdef __SSE2__
> -         else if (!__is_const_known(*this))
> -           {
> -             if constexpr (sizeof(_M_data) == 16 && _Shift > 0)
> -               return reinterpret_cast<_DataType>(
> -                        __builtin_ia32_psrldqi128(__vec_bit_cast<long
> long>(_M_data),
> -                                                  _Shift *
> sizeof(value_type) * 8));
> -             else if constexpr (sizeof(_M_data) == 16 && _Shift < 0)
> -               return reinterpret_cast<_DataType>(
> -                        __builtin_ia32_pslldqi128(__vec_bit_cast<long
> long>(_M_data),
> -                                                  -_Shift *
> sizeof(value_type) * 8));
> -             else if constexpr (sizeof(_M_data) < 16)
> -               {
> -                 auto __x =
> reinterpret_cast<__vec_builtin_type_bytes<long
> long, 16>>(
> -                              __vec_zero_pad_to_16(_M_data));
> -                 if constexpr (_Shift > 0)
> -                   __x = __builtin_ia32_psrldqi128(__x, _Shift *
> sizeof(value_type) * 8);
> -                 else
> -                   __x = __builtin_ia32_pslldqi128(__x, -_Shift *
> sizeof(value_type) * 8);
> -                 return
> _VecOps<_DataType>::_S_extract(__vec_bit_cast<__canon_value_type>(__x));
> -               }
> -           }
> -#endif
> -         return _S_static_permute(*this, [](int __i) consteval {
> -                  int __off = __i + _Shift;
> -                  return __off >= _S_size || __off < 0 ? zero_element :
> __off;
> -                });
> +         else
> +           return _S_static_permute(*this, [](int __i) consteval {
> +                    int __off = __i + _Shift;
> +                    return __off >= _S_size || __off < 0 ? zero_element :
> __off;
> +                  });
>         }
>
>        /** @internal
> diff --git a/libstdc++-v3/testsuite/std/simd/traits_impl.cc b/libstdc++-v3/
> testsuite/std/simd/traits_impl.cc
> index dde41c70aca..9f6e9f42b6e 100644
> --- a/libstdc++-v3/testsuite/std/simd/traits_impl.cc
> +++ b/libstdc++-v3/testsuite/std/simd/traits_impl.cc
> @@ -49,7 +49,7 @@ namespace simd
>        static_assert(__vectorizable<__integer_from<N>>);
>      }
>    template for (constexpr int N : {
> -#ifdef __STDCPP_BFLOAT16_T__
> +#ifdef __STDCPP_FLOAT16_T__
>                                   2,
>  #endif
>                                   4, 8})
> --
> ──────────────────────────────────────────────────────────────────────────
>  Dr. Matthias Kretz                           https://mattkretz.github.io
>  GSI Helmholtz Center for Heavy Ion Research               https://gsi.de
>  std::simd
> ──────────────────────────────────────────────────────────────────────────
>
>
>
>
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